text:
Can instruction be given during a flight conducted under 14 CFR 135?
Let's suppose that a flight is being conducted under 14 CFR 135. The airplane is certified for single pilot operations, but two pilots who are both rated as PIC on the 135 certificate occupy the pilot seats. One of the pilots is a CFI. Does anything in 14 CFR 135 prevent the CFI from giving dual given time to the PIC while he flies?

Sounds like a 135 operator conducting type transition training to me...

I'm not asking the question in that specific context, but that would apply. I'm asking if anything prevents instruction from being given during operations with passengers or cargo while operating under 14 CFR 135.

@RalphJ feel free to make that kind of edit yourself too...

It took me a little bit to figure out the underlying reason for asking this question and it has to do with logging flight time.  For example, in a single pilot operation with an autopilot, the second pilot is not required and therefore cannot log the flight time at all even though their company may want them there.  The FARs don't allow the second pilot to log the time.
The regulation that governs logging of flight time is 61.51.

61.51(h) Logging training time. (1) A person may log training time when that person receives training from an authorized instructor in an aircraft, flight simulator, or flight training device.
  (2) The training time must be logged in a logbook and must:
  (i) Be endorsed in a legible manner by the authorized instructor; and
  (ii) Include a description of the training given, the length of the training lesson, and the authorized instructor's signature, certificate number, and certificate expiration date.

So the question becomes what training was given during the revenue Part 135 flight and were you an authorized instructor?
The term Authorized Instructor is only found in Part 61.

61.1. Authorized instructor means—
  (i) A person who holds a ground instructor certificate issued under part 61 of this chapter and is in compliance with §61.217, when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
  (ii) A person who holds a flight instructor certificate issued under part 61 of this chapter and is in compliance with §61.197, when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
  (iii) A person authorized by the Administrator to provide ground training or flight training under part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.

Since it is a Part 135 revenue flight I would believe the CFI would have to comply with section 135.338 and 135.340.

135.340(a) No certificate holder may use a person nor may any person serve as a flight instructor unless—
  (1) That person has satisfactorily completed initial or transition flight instructor training; and
  (2) Within the preceding 24 calendar months, that person satisfactorily conducts instruction under the observation of an FAA inspector, an operator check airman, or an aircrew designated examiner employed by the operator.

If the authorized instructor has complied with 135.338 and 135.340 they are also bound to the training program for the 135 company.  If that training stipulates you can give dual given during revenue flights than you are more than welcome to do so.  If there is no provision in the training program to provide instruction during revenue flight then it should not be done.

The best option is to request a legal interpretation on the validity of what you are wanting to do.

Short answer: NO
Long answer: In a flight where the CFI is certificated to provide instruction and the two are sitting in pilot seats then the PIC may log any part of that flight as dual instruction.  It does not require the CFI to actually say a word in order for it to count as dual; even if he's merely observing, if something bad happens he'll open his mouth or take controls.
Part 135 is more about requirements for operations; it does not affect flight instruction, which is a part 91 regulated activity.

Nothing here gives any explanation why "no" is the correct answer to the question "Can instruction be given during a {Part 135 flight}?"  Are you answering the question "does anything in Part 135 prevent..."?

You can't log training time unless a CFI endorses your logbook, per 61.51(h), so there's more to it than just being in the aircraft together (related question). And I assume that in your last sentence you meant to say that flight instruction is regulated by part 61 (not part 91)?


text:
What if an airspeed prescribed by a published procedure isn't safe for a particular aircraft's operations?
Departure and arrival procedures often prescribe airspeeds which must be maintained at certain fixes. 14 CFR 91.117 sets rules for airspeeds but has a paragraph which states that the airspeeds don't apply if the aircraft can't operate safely within those limits. Does any regulation or published recommendation alleviate aircraft from the airspeeds prescribed on instrument procedures for similar reasons?

"Unable JEBBB arrival, can't fly that slow"

The most powerful word in the pilot/controller glossary is Unable.

@voretaq7: Ye link breaketh.

https://www.faa.gov/air_traffic/publications/media/pcg_4-03-14.pdf

If you are unable to comply with a published procedure, you simply inform ATC and the controller will tell you what to do instead. The controller will most likely be authorized to make this decision, even if there is no written regulation covering the specific case.

The wording of your question seems to suggest that 91.117 doesn't apply to instrument procedures, but there's nothing in the regulation itself to say that. Specifically, 91.117(d) is very broad (emphasis mine):

If the minimum safe airspeed for any particular operation is greater
  than the maximum speed prescribed in this section, the aircraft may be
  operated at that minimum speed.

Having said that, if you can't comply with any procedure or instruction you should inform ATC and let them tell you what to do. The AIM mentions airspeed in the context of holding (section 5-3-8), but it's reasonable to assume that you should inform ATC in any scenario where you can't comply:

Pilots unable to comply with the maximum airspeed restriction should
  notify ATC

From the ATC side, the ATC orders have an entire section (5-7) on speed adjustments, but one of the first things it says is:

It is the pilot’s responsibility and prerogative to refuse speed
  adjustment that he/she considers excessive or contrary to the
  aircraft’s operating specifications.

So if you can't comply with an airspeed restriction (or any other restriction) just tell ATC "unable" and work it out with them; that's what they expect, based on the ATC orders.

From the same regulations:

§ 91.3 Responsibility and authority of the pilot in command.
(a) The pilot in command of an aircraft is directly responsible for,
  and is the final authority as to, the operation of that aircraft.
(b) In an in-flight emergency requiring immediate action, the pilot
  in command may deviate from any rule of this part to the extent
  required to meet that emergency.
(c) Each pilot in command who deviates from a rule under paragraph (b)
  of this section shall, upon the request of the Administrator, send a
  written report of that deviation to the Administrator.

I'm not sure why emergency authority is relevant here?

Indeed, being unable to comply with an approach clearance is not, in itself, an emergency.

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text:
Are aircraft with squirrel cage fans as wings viable?
I'm having trouble finding it, but a few years back I saw a YouTube video where a gentleman had designed and built an aircraft that used what I can only describe as squirrel cage fans for the upper surface of the wings. As far as I recall he made some bold claims about how efficient and capable this design was.
I was skeptical at the time and haven't seen this design since my original viewing of the video.
Does anyone know anything about this design? If so, is it viable at the scale of your average drone? Is it viable at the scale of a transport category aircraft?

You're not thinking of the Magnus effect are you?

Yea that might be it! I think this is the video. https://www.youtube.com/watch?v=kgOAwzG9Fd0 I'm going to update the question in a little while.

I still think it's safe to link to that Magnus effect question with this one as a duplicate though. Just didn't know what to call this.

The design of a magnus effect device and a fan wing are significantly different as well as the actual physics thereof...I don't see this as a duplicate.

Unfortunately, I can only speak for MAV design, but the lab I work in at the University of Maryland (UMD) has done some work on a similar design called a cyclorotor.  Here's a paper that might have some information for you (https://www.researchgate.net/publication/301680815_Development_of_a_quad_cycloidal-rotor_unmanned_aerial_vehicle?ev=prf_pub), but I'll try to sum up here.
The neat thing about these aircraft is that you have, in theory, a very efficient and simple rotor system.  Instead of a rotor blade where only the outer portion of the blade is moving at a desirable speed, the entire blade is moving as fast as you want it.  Similarly, instead of the complexity of a swashplate and needing to actually tilt a rotor in the direction you wish to travel, you can actuate the pitch of each blade in a cyclorotor arrangement with respect to its location in its rotation (analogous to cyclic blade control) and generate a thrust vector in virtually whatever direction you want...all without tilting the aircraft's body.  I've copied a figure from the paper I linked to illustrate this:

In this image, you have 4 blades rotating around on a drum.  It's not quite the "squirrel cage", but similar.  Each blade is allowed to pitch, as on a Ferris wheel, as it spins around.  To govern this pitching, each blade is attached to an offset bar near the center of the drum's rotation.  Hence, as the blades spin around the drum, their pitch is passively modified, forcing them to generate thrust in a particular direction.  Change the orientation of the offset bar, and the cyclic pitch of the blades changes accordingly, thus modifying the direction of the thrust vector...all without changes the vehicle's flight attitude.  For reference, a helicopter has to tilt over in whatever direction it wants to go (generally speaking), increasing drag area and, based on rotor dynamics as well, limiting its response time.  Plus, it could be handy to keep a stable, level platform that can still move around (say for filming or something like that).  It is a really neat feature to have.
The paper is likewise optimistic about the efficiency of the design.  However, measuring efficiency is a VERY tricky thing to do in a way that you can truly compare two designs.  For example, disc loading is typically measured on a helicopter as the gross weight of the aircraft divided by the rotor disc area.  On a cyclorotor, if you want to compare on that metric...what is the disc area?  The paper utilizes the projected area of the cycloidal rotor (the "drum", if you will, the forms the fan wing concept).  Some argue that utilizing that doesn't give you a true apples-to-apples metric to compare with because well...how IS that projected area and the weight it lifts necessarily comparable to a single-rotor helicopter's disc?  Even for coaxial helicopters, I've heard substantial arguments about how to best reduce a coaxial rotor system to a single rotor system for a true apples-to-apples comparison...and I'm not sure you could find one that involves disc loading for this present case of the cyclorotor.
Power loading (gross weight divided by installed power, so the higher the quotient, the better hover efficiency you have -- more weight for less power) seems like the go-to metric then, since it has less "wiggle room", per se.  However, now that you have that, how do you still say that two helicopters at a given power loading are better or worse than each other?  Power loading doesn't capture everything (only hover efficiency), so an additional parameter is necessary.  What is being held constant between the two designs that gives a basis to say whether or not a given design is actually more efficient than another?  In the UMD paper I've linked to here, it posits disc loading...which, as said, may be a questionable way to do that.  These are the two of the "go-to" parameters of helicopter performance, so we might need to make something new.
Hence, I think that the best answer that I have is that the design is still being felt out, and I'd be wary of people who posit massive efficiency increases.  There is no free lunch, and efficiency is a tricky thing to nail down.  I've seen more CAD of fan-wing designs than I have vehicles (at least, for those larger than the MAV scale), so I'm inclined that think it's not living up to expectations for the larger vehicles, but that's pure opinion.  Also, this said, I know of at least 5 of these vehicles that have been made and flown at UMD, though none over 10 pounds gross weight.

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text:
How does PF or "pilot flying" and PNF or "pilot not flying" affect logging?
How does PF or "pilot flying" and PNF or "pilot not flying" affect logging for pilots operating in a two-pilot environment?
Here are some granular examples...

How should actual instrument be logged? Should a PNF be able to log actual time if they are monitoring during instrument conditions?
How should landings be logged? Should a PNF be able to log landings they were not at the controls for?
How should approaches be logged?

Any insights are appreciated. This question is in reference to FAA regulations only.

While the question is fairly broad, looking at 61.51 is a good place to start, as well as 61.57 for some of the currency stuff.
But let's try and work out some of your examples:
How should actual instrument be logged? Should a PNF be able to log actual time if they are monitoring during instrument conditions?
61.51(g) is what dictates the logging of instrument time:

(1) A person may log instrument time only for that flight time when the person operates the aircraft solely by reference to instruments under actual or simulated instrument flight conditions.
(2) An authorized instructor may log instrument time when conducting instrument flight instruction in actual instrument flight conditions.

So barring any legal interpretations out there or regulations I'm missing (note, I'm not an instrument rated pilot nor attorney), a strict reading of the reg would indicate that only an authorized instructor, conducting instrument flight instruction, can log instrument time when not operating the aircraft.
How should landings be logged? Should a PNF be able to log landings they were not at the controls for?
The short answer is no. Assuming the landings are being logged for currency reasons, 61.57(a)(1)(i) states that the person had to be the sole manipulator of the flight controls. 61.57(b)(1)(i) says the same thing, but for night currency.
How should approaches be logged?
Again, I'm guessing this would be for instrument currency, at which point 61.57(c)(1) would be in play.

Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in an airplane, powered-lift, helicopter, or airship, as appropriate, for the instrument rating privileges to be maintained in actual weather conditions, or under simulated conditions using a view-limiting device that involves having performed the following -
(i) Six instrument approaches.
(ii) Holding procedures and tasks.
(iii) Intercepting and tracking courses through the use of navigational electronic systems.

Again, I'm not an instrument pilot, but despite "sole manipulator of the controls" verbiage being absent in this particular paragraph, I'm guessing the FAA would construe it to be the same.
Of course, I'm more than happy to be corrected should I be wrong.


text:
Has there ever been a US airport with identifier PM2?
I have an old entry in my logbook having landed and departed from an airport with identifier PM2. I think I either can't read my handwriting or I made a mistake because I can find no record of such an airport ever existing. Does anyone have a way of finding out?

It's probably a mistake or bad writing.  Could it be PMZ (KPMZ) in Plymouth NC?  It would help if you could narrow it down geographically.  What was the origin of the flight to it and the destination when departing it?

To expand on Gerry, If you have the flight time and departure/arrival airports, and type of plane, it would help narrow it down a lot.

I think it's PMZ. On 8/6/08 I flew in a 172 from MAO to PM2/PMZ with a duration of 1.8. Then PM2/PMZ to EQY with a duration of 2.2. So I think that solves the mystery.

Here was the flight that day assuming KPMZ. https://skyvector.com/?ll=30.27677933896586,-81.80566405639985&chart=301&zoom=12&fpl=%20KHEG%20KMAO%20KPMZ%20KEQY%20KHEG

No. PM2 would not be a valid ID in the FAA system. According to the FAA order defining the system:

1−2−7. ASSIGNMENT SYSTEM §c. Most one−number, two−letter identifiers have been assigned to aviation weather reporting and observation stations and special−use locations. Some of these identifiers may be assigned to public−use landing facilities within the United States and its jurisdictions, which do not meet the requirements for identifiers in the three−letter series. In this identifier series, the number is always in the first position of the three−character combination.
  (emphasis mine)

So two letters followed by a number is not a valid ID.

Can you provide a FAA number and/or link for said order?

@UnrecognizedFallingObject Aw dangit! That was supposed to be a link. Gimme a minute and I’ll put it in.


text:
How are the hours that count toward fatigue limits calculated by the FAA?
Which part of the flight actually counts toward the flight hours limited by the FAA? Is there any accounting for the time spent in flight preparation in so far as compensation or hour limits?

Thought this was referring to metal fatigue!

Flight prep and other non-flying duties are accounted for, but not under “flight time.” There are seperate regulations for time “on-duty” and time actually flying.
Flight Duty Period
The flight duty period begins when a flightcrew member is required to report for duty with the intention of conducting a flight and ends when the aircraft is parked after the last flight. It includes the period of time before a flight or between flights that a pilot is working without an intervening rest period. Flight duty includes deadhead transportation, training in an aircraft or flight simulator, and airport standby or reserve duty if these tasks occur before a flight or between flights without an intervening required rest period.
Flight Time
The regulations define flight time as: when the plane is moving under its own power before, during or after flight.
Rest period
Rest periods are “measured from the time the flightcrew member is released from duty.”
Cumulative hours
Cumulative hours are limited for both flight duty period and flight time. There are 7 day and 28 day limits on FDP; there are 28 day and 365 day limits on block time. The 7-day and 28-day limits are expressed as hours, so a rolling 168 hour limit (7 days) and rolling 672 hour limit (28 days). [thanks Ralph J]

This info was taken from the FAA Fact Sheet – Pilot Fatigue Rule Comparison

@RalphJ Thanks. Edited that into my answer.


text:
Do the elevators generate vortices?
I want to know whether the elevators generate vortices like the wings do? If yes, then where do the vortices form?

Yes, they do. The elevator vortices I have observed are pinned to the trailing edge corners of the elevator and are made visible when the aircraft is traveling through moist air that is very close to the dew point temperature, and at a significant angle of attack. There's a hotel at LAX where you can look out the windows and watch planes fly by on final, just before touchdown; on a foggy afternoon they are all trailing vortices behind them.

Proud Bird restaurant just down the road from LAX is another good place to watch from. On Aviation Blvd, very close to the landing ends of 24 L/R and 25 L/R.  Hilton at LAX also has a good view, especially from the upper rooms facing south. Farther away tho.  Too dry when I was there a month ago, no vortices seen.

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text:
Did Boeing need MCAS because the 737 MAX didn’t meet FAA longitudinal stability requirements, or was it to make the MAX feel like the NG?
I’m confused on this. I’ve read in multiple different places that

MCAS was required due to the easier stick forces at a high angle of attack and a high G force, and because of this, the Max didn’t meet FAA longitudinal stability

Others have said that

The Max has MCAS because they wanted it to feel like the NG

Confirming the easier forces, but excluding the FAA longitudinal stability possibility. Almost every source contradicts another one, and I’m unable to locate any info through the FAA site.

The Boeing 737 MAX MCAS system is there ONLY to meet the FAA longitudinal stability requirements as specified in FAR Section 25.173, and in particular part (c) which mandates "stick force vs speed curve:, and also FAR Section 25.203 — "Stall characteristics".
FEDERAL AVIATION REGULATIONS Sec. 25.173 — Static longitudinal stability.

Sec. 25.173 — Static longitudinal stability.
Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows:
(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or V FC /M FC,whichever is appropriate, or lower than the minimum speed for steady unstalled flight.
(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.
(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.
(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.

FEDERAL AVIATION REGULATIONS Sec. 25.203 — Stall characteristics.

Sec. 25.203 — Stall characteristics.
(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.
(b) For level wing stalls, the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees.
(c) For turning flight stalls, the action of the airplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect a prompt recovery and to regain control of the airplane. The maximum bank angle that occurs during the recovery may not exceed—
(1) Approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction, for deceleration rates up to 1 knot per second; and
(2) Approximately 90 degrees in the original direction of the turn, or 60 degrees in the opposite direction, for deceleration rates in excess of 1 knot per second.

Here is some additional information: http://www.b737.org.uk/mcas.htm

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.
The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

I doubt the MAX saga has anything to do with static long stab.

From publicly available sources, the first motive of the MCAS was to satisfy the stick force per G requirements, or maneuvering stability (not static longitudinal stability, which deals with speed stability). This is confusingly captured in 14 CFR 25.255(b) and (c) under Out-of-Trim Characteristics, but also applies broadly to buffet characterization at mid/high Mach (see AC 25-7C 25.251):

(b) In the out-of-trim condition specified in paragraph (a) of this
  section, when the normal acceleration is varied from + 1 g to the
  positive and negative values specified in paragraph (c) of this
  section—
(1) The stick force vs. g curve must have a positive slope at any
  speed up to and including VFC/MFC; and
(2) At speeds between VFC/MFC and VDF/MDF the direction of the primary
  longitudinal control force may not reverse.
(c) Except as provided in paragraphs (d) and (e) of this section,
  compliance with the provisions of paragraph (a) of this section must
  be demonstrated in flight over the acceleration range—
(1) −1 g to + 2.5 g; or
(2) 0 g to 2.0 g, and extrapolating by an acceptable method to −1 g
  and + 2.5 g.

This is only required for Mach 0.6 and above, since this is where transonic effect sets in. And indeed, according to the Seattle Times report, the shock formation induced from the pylon/wing junction made the maneuvering characteristics uncertifiable, hence the conceptualization of MCAS. Note that MCAS stands for Maneuvering Characteristics Augmentation System, and it originally required the detection of high load factor, as well as high AOA in one of the vanes, in order to activate.
Later on in the flight test program, there were further complications from the nacelles in the low-speed stall evaluation, which is correctly cited in @MikeSowsun's answer (required by 25.203). However, stall evaluation, unlike stick force/G evaluation, occurs at 1G (or 1.15G in turning stall); therefore, the G load activation requirement was removed, and MCAS became a single source activation feature.

The MAX feels like the NG, which feels like the Classic, which feels like the -200. The flight control artificial forces are identical across the different versions, which share a type rating. No need for MCAS there.
Also, the MAX is longitudinally stable. MCAS was implemented for tackling some situations in extreme corners of the flight envelope, as mentioned in this answer and a few others.

Just one tiny note:  any B737-900/-800/-700/-600/-500/-400/-300/-200/-100 is considered by BCA a “Classic” since this term applies to any airplane no longer in production.

@CarloFelicione Then please edit the 737 tag wiki on this site, and the Wikipedia pages for B737.


text:
Why do flight attendants on the 787 lock the dimmable windows at different phases of the flight?
I’ve heard of many passengers complaining about the FAs locking the dim setting at different phases of the flight. Seems to be something the passengers definitely don’t like. And the passengers cannot change it. What is the reason for locking the window brightnesses?

Do you mean for takeoff and landing? If so, this should answer your question: Why open up the window shades before takeoff and landing?

@Bianfable while I have heard it for takeoff and landings, a vast majority also say during at cruise altitude. Others have said you can avoid it by suggesting your setting when the FA’s ask, but I don’t see why they would do that when it just defeats the entire purpose.

I don't understand the meaning of "dimmable".  Please explain more.  Is this some sort of electronic control over how transparent the window is?  Does it involve a physical barrier (window shade), or not?  A few extra words would help some of us understand the question much better, thank you.  Or, I could always ask ASE in a separate question.

The 787 has electronically-dimmable windows (I believe they use liquid crystals) to change how opaque they are without a physical shutter.

@quietflyer The 787 has electrochromic windows (sometimes called "smart glass.") There's a control beneath each window where the passenger can change its opacity, but the cabin crew can override the controls and make them all transparent or opaque during flight phases when all of the windows need to be open or closed. There are no actual physical shades like on most other passenger aircraft.

@quietflyer You can see them in action in this video. Note how slowly the transparency changes, even though the film's author says "instantly" ;-)

I found it distressing that I could see the sun but the sky was black. Dimmable windows were my theorized explanation but this confirms it. I saw the sun but the sky was black and the sun had a pink colored aura. I saw an atoll but couldn't photograph it.

On long-haul flights, most passengers will want to sleep, often including when the aircraft is flying through daylight. It's especially common on East-West long-haul flights for lots of people to want to sleep when it's daylight outside the aircraft, since the aircraft will often be crossing many time zones (and, during the summer months, also often flying through regions where it's daylight all day or nearly all day.)
Even on aircraft without dimmable windows, the cabin crew will normally tell passengers to keep window shades closed after the first meal service when the cabin lights are turned out for that reason. On the 787, the crew will dim and lock all of the windows centrally instead of telling the individual passengers to do it and this also prevents a passenger from undimming it and waking everyone up. Unfortunately, that does happen from time to time on aircraft with normal window shades where the crew can't prevent it and it is really annoying when you're trying to sleep if you don't have an eyemask on.
Here are some common examples of the types of long-haul flights where many or most passengers will be trying to sleep during hours where the aircraft is flying through daylight:

Flights from East Asia to the U.S., even though they're typically at least nominally "overnight" flights, will actually spend much of the time flying through daylight. The actual darkness outside will only last a fraction of its usual time, since the aircraft is flying in the same direction as the Earth's rotation at a pretty significant fraction of Earth's rotational velocity.
Flights from Europe to North American typically occur entirely during daylight. While it's 'daytime' on these flights, these days are often (quite literally) long days for travelers, so many will want to try to nap during the middle of the flight.
Late flights from North America to Europe often leave North America within an hour or two of midnight and land in Europe around midday, even though it's only 6-8 hours later. The latter half of the flight takes place in daylight, but passengers will still be trying to sleep.
Summertime flights from North America to East Asia will typically experience midnight sun. Even though these flights often occur almost entirely during 'overnight' hours, their flight paths frequently take them well into the Arctic Circle — sometimes even near the North Pole — where it will be daylight outside, even though it may be near midnight local time. This typically occurs around the middle of the flight, when nearly everyone will be trying to sleep. As most readers of this site will know, the reason these flights fly so far North is because that's actually the shortest path, since Earth is a spheroid. For example, this map shows the shortest-distance path from New York's Kennedy Airport (JFK/KJFK) to Shanghai's Pudong Airport (PVG/ZSPD):

Shortest path from JFK to PVG, generated on gcmap.com

to be exact, at latitudes beyond arctic circle, during summer there will be daylight 24 hours a day, as for a certain period of days depending on how high a latitude you are at, the sun will not set at all. Flying at high altitudes will make this better or worse, depending on whether you do or do not (respectively) like sunlight ;)

I've known them want the shades down even on fully daytime flights. Sleeping pax are less likely to press the call button.

I get very frustrated with this custom. I fly business or first class on long haul flights, and the amenity pack always includes eye shades. Why do the passengers who want to sleep not use them? My objective on a long haul flight is to adjust as quickly and completely as possible to the new time zone, and exposure to natural daylight when it is daytime there is the best way of doing that.

@PatriciaShanahan By nature of being part of the near-disposable amenity kit, the masks are "economy" quality, and I have yet to find a pair that fits comfortably on my face.

Also, the majority of the passengers are in economy, where the amenity kits typically aren't provided and it's already much harder to sleep, due to sitting with a slight recline rather than lying down in a flat-bed seat.

I would be really annoyed if I didn't have the freedom to look out the windows, take pictures, etc. whenever I wanted.  I mean I can understand other people wanting to sleep, but I have learned that it is easier to adapt myself to sleeping (covering my eyes with something dark, wearing earplugs, etc.) rather than try to get others to conform to my expectations of quiet and darkness.

For takeoff and landing, @bianfable pointed out the answer. For longer flights across timezones, they set them to dark in order to not disturb passengers who would like to sleep.
And yes, I personally hate it when they do that and avoid the 787 whenever possible.

Or in my experience, at least, the people who'd rather watch the TV screens.

I hate the B787's electrochromic windows because they change color sooo slowly. When I see something interesting outside, it's impossible to untint the window quickly to take a good quality photo!


text:
How do flaps help an aircraft take off at a lower speed, yet cause drag at the same time?
Wouldn’t the drag caused by the flaps just decrease the acceleration, so, although they can lift off at a lower speed, wouldn’t it be faster to just use no flaps and rotate at the higher speed that is required?

Note that usually flaps can be extended to different settings. The one used for takeoff create less drag than the one used for landing. Moreover, while on ground, you may take into account tyres and wheels restrictions and other acceleration penalties due to undercarriage.

This question needs some help.  The question is different from the title.  The question appears to be asking whether it would take less time (not distance) to leave the ground with flaps up.  The title is asking something completely different.  Please fix!

Great answers but...  see above.

The take-off technique for the Me-262 was to accelerate with flaps up and to lower them only at the take-off speed. That opened up the use of runways which were nominally too short for a standard take-off with flaps set to 20°. Your reasoning is correct, but when something fails, you are in deep trouble.

Increasing the flaps does increase the drag, but not by that much initially. For the first stages of flaps you gain more by reducing required takeoff speed. If you would increase the flaps more and more, eventually the drag would become too much and you would lose takeoff distance again.

Flap setting has an affect on the wing’s lift coefficient and on the aerodynamic drag. Increasing flap angle increases the lift coefficient, and therefore reduces stalling speed and the required takeoff speed (the same lift will be created at smaller air speed due to greater lift coefficient). This reduces the takeoff distance. In the same time increased flap angle increases drag, reduces acceleration, and increases the takeoff distance.
The net effect is that takeoff distance will decrease with increase of flap angle initially, but above a certain flap angle the takeoff distance will increase again. An optimum takeoff setting can be determined for each type of aircraft and any deviation from this setting will give an increase in the takeoff distance.
The flap setting also affects the climb gradient. Increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass. If there are obstacles to be considered in the takeoff flight path, the flap setting that gives the shortest takeoff distance may not give the required climb gradient for obstacle clearance.

(skybrary.aero)
Note that the climb gradient is also affected:

More flaps allows taking off from shorter runways, but reduces obstacle clearance capabilities.

I would suggest that sometimes the climb gradient is better with some amount of flaps deployed.

I might be wrong about that-- at least for no wind case.  I think is surely true sometimes w/ strong headwind.

Downvote for asserting that climb gradient is worse with flaps extended. It's almost always better (of course only up to a certain flap extension angle). Climb rate is usually best with flaps fully retracted, if the comparison is made at a steady-state condition rather than during acceleration.

@pericynthion Did you read the page I quoted? "Increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass."

But your image claims "less flaps -> better climb gradient" and vice versa, which is false.

@pericynthion I tried to find another reference for that in my 737 FCOM, but didn't find anything. Have a look at this YouTube video by a real world 737 pilot. He explains the different climb gradients for different flap settings quite well.

You wrote " Increasing flap angle increases the lift coefficient, and therefore reduces stalling speed" which is not really true. What it does is increases $c_{L_{max}}$, and therefore it decreases stalling speed. You can increase the lift coefficient just by increasing the angle of attack on a wing with no flaps.

@MikeY No, I did not write that. The page I linked wrote that, not me.

OK, on my home monitor the yellow blended in to white. :)  What you quoted isn't quite right, though.

Instead of "better climb gradient" and "worse climb gradient", an improvement here would be using more objective and common phrases like "increased climb angle" and "decreased climb angle." Better and worse are subjective terms and depend on what one is trying to accomplish. Sometimes a decreased climb angle results in a faster climb and that's better.

Because without flaps extended there is less safety margin for stalling in the landing.

Picture above from this answer shows the lift coefficient as function of the angle of attack. With flaps extended, a certain amount of lift is reached at a lower AoA than without flaps.
It is a safety feature during landing, when speed needs to be reduced as much as possible. Not so much during take-off, and TO flaps setting is often not fully deployed. But deploying them does reduce TO distance since the same lift can be reached at a lower speed.

The graphic is deceptive, as typically when you lower flaps on an airfoil (no slats) while $c_{L_{max}}$ increases, the AOA at which you hit  $c_{L_{max}}$ decreases. Your plot shows the AOA for  $c_{L_{max}}$ increasing. Nice article showing this result. http://n91cz.com/AOA/AoA_Article_Web.pdf?fbclid=IwAR02csvjmUQYEWd4RgApdXDhAmWIW_WRiWxyVc555xHYsT-oVu_F8fsB8LY

@MikeY Yes indeed.

Wouldn't it be better to use no flaps, and rotate at the higher speed that is required?
Well, now we are talking about a short field take off.  If you ever watched Lindbergh's takeoff the day of the famous journey across the Atlantic non-stop flight, barely clearing a power line on climb out, or even actually done one off a muddy or snowy runway, you know the answer to this question.
Many take-offs are done with flaps up because an aircraft will produce more lift per given amount of thrust (drag) " clean ".  Key to this is reaching the speed where the wing can be set to its optimal Lift to Drag ratio AOA and also be going fast enough to produce adequate lift.
Cambering up a wing makes it a better lifter, giving the ability to get the plane off the ground at a lower speed, but at the expense of more drag (read thrust and drag interchangably).
Notice the same effect can be achieved by increasing the coefficient of lift of an unflapped wing by raising the AOA past optimal L/D ratio, but the risk is, especially with wings that stall at a lower AOA, the power on stall!
So if the runway is covered with take-off roll drag producing items such as mud or snow, or simply is not long enough to reach flaps up takeoff speed, we use some flaps and slats to get airborne and climbing as soon as possible.
Once airborne, it is generally best to retract flaps and climb out Vy, unless that obstacle needs to be cleared, then we use the less efficient Vx.
Increasing camber (and AOA) does increase amount of lift at a given speed, but will require a proportionally greater amount of thrust.  A bigger or turbocharged engine helps here, along with careful monitoring of airspeed.

Note: flaps make high AOA, low speed flight slightly safer by stalling the wing near the root first, similar to dropping leading edge slats near the wing tips, or having "washout" built into the wing.

Once you get off the ground, you can accelerate much faster than when the wheels are down there in the dirt, grass, or whatever.  Even on a paved runway, the tires create quite a bit of rolling resistance, as anyone who's moved a light plane by hand should realize.

That's true.  Watching the Lindbergh film, they towed the plane out with 2 cars, so I wonder if ...

"Most take-offs are done with flaps up." Jetliner takeoffs are done with the flaps lowered to a takeoff setting, typically around 10 degrees.


text:
If the pilots used the brakes upon landing, would the force essentially slam the front wheel down?
So if the pilots braked immediately, with only the back wheels down, and nose wheel still up, would the force from braking cause the front wheel to come slamming down?

Not in most aircraft.  For the braking force to be high, there needs to be a substantial load on the mains, so the nosewheel is close to settling.  Also, generally technique on mid to small sized aircraft is to hold the nose off until the speed is lower, where it will more naturally drop.  For max braking, many aircraft will use the technique of copious rear elevator (nose up pitch) which transfers weight to the mains.  Different aircraft have different design trades, different tail clearances, and different operational recommendations.

If you're a reasonably skilled pilot, you adjust your braking effort and elevator so that that doesn't happen.  Same as you generally don't slam on the brakes when you're driving a car.

It would be nice if an answer addressed the general case, rather than transport aircraft.  There are lots of utility aircraft, and light aircraft and even military and cargo aircraft with different brakes, different aerodynamics, different braking systems, and of course different recommended techniques.  Having answers which are focused on transport aircraft with highly specialized systems to manage secondary effects tends to obscure the underlying understanding of how the brakes, the aircraft mass distribution and the underlying issues the OP appears to be seeking to address.

If you get on the brakes hard after main touchdown, once the anti-skid is active following wheel spin-up (you can't land with brakes applied on any airliner with anti-skid - brakes are depressurized until after the main wheels start to spin and/or you are weight-on-wheels for a minimum time) and don't apply any additional compensating elevator, the nose will come down faster than if just hold the existing elevator input, and will contact a bit harder, but "slamming down" would be a bit extreme unless you also relieved some of the control back pressure as you did so. Then it would be a bit firm and if you were reckless enough you might blow the nose tires, and somebody would have a chat with you.
In practice though, on most jets you land the mains, then the nose in two separate actions and during that "second phase" you are applying elevator to modulate the sink rate of the nose to make it touch down reasonably gently for that "2nd landing".  If you were on the brakes hard at that point you would normally just compensate instinctively with extra elevator input to get a gentle landing of the nose.
You would certainly have enough tail power available to counteract the nose down torque of the braking at high speed.  But you would probably get scolded for getting on the brakes before nose wheel contact anyway, and a lot of pilots will keep their feet low on the pedals to avoid applying brake, then move them up once the nose wheel is down.

Yes. Hammering on the brakes right at touchdown will apply a torque to the airplane which will tend to rotate the fuselage "nose down".

Yes, it would. In the case of the A320, you would actually put the nose down smoothly (fly it down). If you hold the nose up and apply harsh brakes (or have Med autobrake set), then the nose wheel will slam down eventually.


text:
Why are emergency vehicles needed for a runway excursion?
When passenger jets have a runway excursion incident such as this one in Birmingham it is apparently common practice to send emergency services vehicles to the aircraft.

Why are emergency services vehicles necessary?
I would think the plane is more than capable of getting back onto the Tarmac, so why doesn't it?

It is more than capable of getting back onto the Tarmac - really?  What makes you think that?

Something to do with 2 big jet engines?

Ever tried to use that much force to dislodge a main wheel stuck in mud while the other one is on tarmac? What happens when the stuck wheel comes free?

Clearly I'm missing the point, what's the issue? We got planes taking off on ice runways, planes even reversing themselves but comes off a taxiway and this?

There is no way the insurance company would be happy going full throttle in the dirt with a commercial jet... Think of all the FOD that would be created, both on the aircraft and possibly back on the runway. Just tow it out.

What you can't see is the wheels on the grass buried up to the bogies.  The wheels are relatively narrow and not designed for "off road" use.  45 tons of aircraft will quickly sink it.  Even if you could get enough power to move it, the manoeuvre will almost certainly cause a twisting moment on the undercarriage which it is just not designed to handle. Even assuming no twisting, the sudden lurch as it freed itself would likely damage the nose leg.  I've seen a heavy jet on the grass.  The only way to move it is to tow it.

Here you can see how buried the wheels were.  To answer your question about emergency services, imagine an hydraulic line had split and sprayed oil onto the hot brakes (it had just landed, and this has happened before).  A fire starts.  If the emergency services had not deployed, what would your question be now?

In addition to the above, most of the equipment in that photo is airport based. They exist literally for this type of work - why WOULDN'T you use caution and involve them?

I'd also say that the emergency services are there more because its a very efficient portable hose to wash the dirt off the runway/undercarriage after they pull it back out. You can see that happening in the link @Simon provided.

@Dan Exactly. You have people trained in dealing with aircraft emergencies just sitting there. You might as well have them next to the aircraft instead of sipping tea on the other side of the field.

cool pictures, @Simon, but it's hard to tell just how deeply the wheels had sunk. The plane does look like it's left wing down, but that could be the slope of the grass. Am I missing something else?

@FreeMan Click the link in my comment 5 above...

@Simon I did, I just didn't see anything that indicated buried bogeys. Not doubting you, just sad I couldn't see it up to its knees in mud, as it were. :)

You need to know because?

Why are emergency service vehicles necessary?

Something might have been damaged. What if something was and there is a fuel leak somewhere? Hydraulic leaks are also dangerous, and more likely as hydraulic lines run to the brakes and nose wheel steering. Better safe than sorry, so emergency services go check.
Sometimes, they also disembark passengers via the ladder if it looks like towing the aircraft to terminal will take too long and the aircraft is on unpaved surface where the mobile stairs have trouble going.
They can also help with moving the plane, especially on smaller airports that only have one runway and so have to close until the plane is moved away anyway.

I would think the plane is more than capable of getting back onto the Tarmac, so why doesn't it?

No, it isn't. Aircraft not designed for unpaved surface will usually sink in somewhat, especially if the ground is wet, and require a lot of power to pull out. Sometimes, the tow truck is enough to get it out, sometimes they have to put planks under the wheels and sometimes they have to get out hydraulic jacks or inflatable support bags.
Even if the plane did not dig in and is easy to move, providing the power with the engines is not a smart thing to do, because the engines will throw up dirt and stones that could damage them, other parts of the aircraft, or something else around. Tow truck is much better for that job.

Going off the the runway can be pretty rough. It not unlikely that a passenger got injured or part of the plane got damaged.
Going full throttle on the jets will rip up the dirt and grass and fling it everywhere. It will also put quite a bit of force onto the landing gear which already got a beating from going into the grass.
Much safer to tow it back. You can see the white towing vehicle in front of the plane.

And even if the risk of someone being injured wasn't likely, you already have the emergency vehicles available so you lose nothing by sending them. Best case: Your crews get an impromptu drill and something to make their day more interesting. Worst case: you need them there ASAP as the aircraft bursts into flames. Either way, there's no downside to sending the emergency services instantly.


text:
Before COVID, what was the largest aviation crisis ever?
The Covid-19 pandemic is the worst crisis aviation has ever faced. Which one was the biggest one before that pandemic?

could you define "biggest"?

@Federico, w.r.t. impact, e.g. airplanes grounded, airliners struggling, employees getting laid off, passengers afraid to fly, and so on. I exclude World Wars periods.

I'm not sure the Covid-19 pandemic is a crisis to aviation. Most certainly it is to the airline industry, but to aviation as a whole? Really? My father has flown his Cub weekly all summer.

To commercial aviation I meant @CGCampbell.

Crisis is somewhat hard to define explicitly but the September 11th Attacks had an enormous impact on the commercial aviation industry in perhaps more ways than the current Pandemic has.
It took nearly 3 years for aviation travel numbers to return after the attacks and arguably we are still seeing lasting impacts on changes to broad aviation travel procedures as a direct results of the attacks even 20 years later.

Probably the right answer, maybe I was too young back then, thanks.

I am skeptical.  Surely the closure of routes and repurposing of personnel, aircraft, and production lines during World War II was a bigger (as a proportion) and longer lasting change, even if aviation was much smaller then.

This was asked a couple months ago.

@JohnK let's close it as a dupe then!

@AnonymousPhysicist I would believe the opposite to be true. The world didn't have many commercial aviation planes or routes prior to WWII. WWII itself advanced the knowledge and equipment of flying to such a great extent that I am a firm believer that if WWII hadn't happened, aviation as we know it would have been at least a decade longer in happening.

Hindenburg disaster may be a candidate. This fire firmly ended the era of the Zeppelins and other great airships filled up with hydrogen that were flying transatlantic routes at that time. Helium filled airships, even if much less flammable, somehow shared the decline as well.
The aviation needed to switching into very different fixed wing aircraft, using technologies developed during WWII.

Ah very interesting answer as well, thanks!


text:
How do paper airplanes create lift if their wings are flat?
Paper airplanes' wings are completely flat, unlike the droplet shaped curvature of a full sized wing. How does a flat wing generate any lift if both sides have the same air pressure?

Even if you cut a wing out of a piece of flat cardboard it glides nicely.

Related: How do I explain what makes an airplane fly to a non-technical person?

The sides don't have the same air pressure, because complex fluid dynamics results in airflow which is faster on top than below when the wing hits the air at an angle (which it does). You can play around on this NASA applet to play with wing shapes and actual aerodynamic equations (simplified to ignore viscosity, but still far more accurate than popular depictions)

I think it might be due to the more Newtonian theory of reaction lift on the lower side of the wing.

Simple: both sides to not have the same air pressure.

@FreeMan: The folds along leading edges or anything are totally irrelevant. Flat wings do create lift just fine (they just have a lower critical AoA and lower maximum $C_L$).

If they could get a washing machine to fly, my Jimmy could land it.

Some simple control-line model airplanes have wings cut from flat sheets of plywood.  Planes with cambered airfoils with convex top sides and concave undersides are still capable of sustained inverted flight, though not very efficiently.  Example: "Radian" radio-controlled motorglider model airplane.

Paper airplanes create lift just like any other airplane. Don't let the wing cross section confuse you; what the air "sees" is a different shape.
When a flat plate flows through air at a positive angle of attack, the stagnation point of the flow (where air splits into an upper and a lower flow path) sits slightly below the forward edge of the wing. Now the streamline just above the one which hits the stagnation point has to negotiate the sharp corner of the leading edge, which will cause it to separate from the surface. This creates a small separation bubble which to the streamlines further up looks like a round leading edge, and since the rest of the wing has no curvature, the flow will attach again soon. The bigger the angle of attack gets, the farther back the stagnation point will be and the bigger the separation bubble becomes. At some point, which is promoted by leading edge sweep, the separated flow will not reattach, but produce a vortex, like the vortex which provides lift on delta wings.

The picture above is from a simulation; see the full video here. Note the red color on the forward part of the upper side: This indicates low local pressure, which is needed to make the outer air negotiate the leading edge plus the separation bubble (see here for an explanation on the molecular level). In the separation bubble, low pressure is not accompanied by high flow speed since the separation turbulence has eaten up most of the kinetic energy.
The actual nose radius of the paper wing does not matter, and it might even be zero (like when you fit razor blades to the leading edge): The air will create its own airfoil shape by padding with separated air what cannot be reached with attached flow.
Paper airplanes have a low enough wing loading that their angle of attack is low and the separation does not become too big. All it needs is a center of gravity at the quarter chord of the wing; that is why you need to fold the piece of paper first.
It is interesting to watch a slightly tail-heavy, delta-shaped paper airplane fly: It will gradually reduce speed and will start to rock its wings when it approaches stall. Once one wing is going down, the sudden increase in its local AoA will increase lift so much that the rolling motion reverses, without noticeable yawing. The delta-shaped flat plate has effective roll damping right into the stall. The lack of yaw is mostly due to the high yaw inertia, but indicates also that drag buildup is moderate at best.
Of course, after a few periods of rocking the nose will drop and it will pick up speed like any well-behaved airplane.

What happens with an (obviously unphysical) zero-thickness plate, or with a rounded edge (either of which mitigate the sharp corner)? Same effect?

@cpast: I tried to be more explicit; let me know if something is missing now.

Let's not forget vortex lift.  The paper airplane you hold in your hand and the Concorde have much in common!  Indeed, issues of bending under aerodynamic stress have led to the strong delta design in paper as well.

And to answer @cpast, increasing the angle of attack causes airflow separation in flat plates ( which can be a horizontal stabilizer) as well as "classic" wings.  Very good early NACA films on You Tube showing this.  A rounded edge, which can be found on a fully symmetric airfoil (which can be a horizontal stabilizer or wing) modifies the effect (beneficially), but the principle of lift creation remains the same.

The gigantic vertical keel on most paper airplanes probably also has something to do with the lack of yawing.

How does a flat wing generate any lift if both sides have the same air pressure?

The two sides do not have the same pressure.
A flat plane can still produce lift (think of putting a plate out of the window of a moving car, if you point it slightly upwards it will strongly push upwards, that's lift). The main reason is the Kutta condition that must be satisfied at the trailing edge: this will create circulation around the flat plate, generate a pressure difference and thus generate lift.
Paper airplanes will inevitably come down not because they do not produce lift, but because they also have drag (and no thrust), that will slow them down and hence reduce the lift generated.

It's funny how many people misunderstand such a fundamental principle of flight like how an aircraft wing creates lift.
Most misunderstandings stem from incorrect textbooks written by unqualified people who know about flying, but not about physics. This incorrect information is often passed on at flight training schools by flight instructors who were themselves given wrong information.
Your question demonstrates this point exactly. I could reword the question to state: "If a paper aeroplane flies with straight wings, does this mean that the shape of an aircraft wing is not the thing that actually generates lift?". The answer is self-evident.
The incorrect analysis of lift usually goes something like this: an aircraft wing creates lift because the air travelling over the curved surface on the top must travel faster than the air moving along the shorter surface underneath. In accordance with the Benouli Principle, the faster moving air creates a low pressure area that sucks the wings upwards.
How then, does an aircraft fly up-side-down? Wouldn't the low pressure area now be underneath, pulling the aircraft towards the ground? How does a paper aeroplane fly with a flat wing? The answer is that it is not the shape of the aircraft wing that generates the lift.
An aircraft wing generates lift due to the physical reactions of the wing surface coming into contact with the air as the wing moves through the air, deflecting air down. In accordance with Newton's Third Law of Motion, that downward force exerts an equal and opposite upward force on the wing, creating lift. Increase the angle of deflection caused by the wing (known as the Angle of Attack) and the force generated increases, creating even more lift at a given airspeed, at least until it reaches the critical Angle of Attack, where the slowing force created by the increased drag eventually exceeds the lifting force, causing the aircraft to stall.
So why aren't aircraft wings flat? As in nature (in the case of birds), the curved surface of an aircraft wing helps to reduce drag, allowing air to flow smoothly over the wing without separating and becoming turbulent. Lower drag means a more efficient wing.
If you want to know more, NASA has some good information on this topic on their website. You can check it out here: https://www.grc.nasa.gov/www/k-12/airplane/lift1.html

probably the best answer.

Because it takes the path of least resistance.  There is lot less drag in slipping forward through the airmass as it falls than in falling flat towards the earth.
Think about it:  If you stuck a putty knife in a block of jello at a 45 degree angle, then pulled it straight towards you, would it come straight towards you, or might the blade tend to angle towards one side or the other?  (depending on which direction the closer edge was...)
Would you say that this phenomenon is due to a differential in gelatinous pressure, or that the putty knife produced lift?  I wouldn't.  I'm not an engineer, but I would say that the knife simply took the path of least resistance.

It is true that a flat wing can produce lift, but a paper airplane's wings are not flat on top. They have a folded piece on top that bows slightly up. Air has to flow a longer distance over that slight curve as compared to the bottom of the wing, just like an airplane. The faster air causes a pressure drop relative to the bottom of the wing, thus lift.

You seem to be falling for the equal transit time fallacy, https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

Here is the new URL for Equal Transit Theory from NASA: https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw1/


text:
What is the minimum allowed altitude for helicopters over a residential area in the US?
I observed a helicopter hovering over a residential area/houses for more than 20-30 minutes. I would guesstimate about 500 feet altitude or more for the helicopter. The low altitude and long duration of the hovering created lots of what I would call unpleasant "noise pollution".
What is the FAA minimum allowed altitude for helicopters over a residential area?

re "possible duplicate":
@fooot I already googled and read that entire question. That question is asking about "when is a Helicopter legally allowed by FAA to fly below the FAA's minimum allowed safe altitude?" and that question is even more specific to a specific situation observed by the OP. They both have wording that talks about minimum altitude but are not the same question.
As others have noted in the comments, they agree this is not a duplicate.

Possible duplicate of Under what conditions would a helicopter be allowed to fly at low altitude over populated areas?

@fooot I already googled and read that entire question. That question is asking about "when is a Helicopter legally allowed by FAA to fly below the FAA's minimum allowed safe altitude?" and that question is even more specific to a specific situation observed by the OP. They both have wording that talks about minimum altitude but are not the same question.

I think @TrevorBoydSmith has a point.  This is a more general question about what is allowed in normal circumstances, not during an emergency.  The question isn't a duplicate.

Agree - not a duplicate. This is actually a much better question than the other one is.

I would like to add to this question: My residential neighborhood has a hospital. The hospital fought in the courts to get a helipad. If the helicopter has to be 500', how does it navigate the residential areas to get to the helipad?

unless necessary to safeguard human life @user3344003 see my comment on @rbp's answer for a link.

In this case, the helicopters are being used for intra-hospital transport, not emergency (i.e., not a trauma center) so its never necessary for safeguard life. Currently they land a distance away and use a vehicle.

It depends on where it is.
A residential area/houses may or not be "congested" -- that term is not defined. But we typically use 500 feet for helis.
When hovering in an area for a specific purpose, such as photo shoots or other surveillance operation, or for some other operational reason, then we can use a lower altitude.

Sec. 91.119
Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an
aircraft below the following altitudes:
(a) Anywhere. An altitude
allowing, if a power unit fails, an emergency landing without undue
hazard to persons or property on the surface.
(b) Over congested
areas. Over any congested area of a city, town, or settlement, or over
any open air assembly of persons, an altitude of 1,000 feet above the
highest obstacle within a horizontal radius of 2,000 feet of the
aircraft.
(c) Over other than congested areas. An altitude of 500 feet
above the surface, except over open water or sparsely populated areas.
In those cases, the aircraft may not be operated closer than 500 feet
to any person, vessel, vehicle, or structure.
(d) Helicopters,
powered parachutes, and weight-shift-control aircraft. If the
operation is conducted without hazard to persons or property on the
surface--
(1) A helicopter may be operated at less than the minimums
prescribed in paragraph (b) or (c) of this section, provided each
person operating the helicopter complies with any routes or altitudes
specifically prescribed for helicopters by the FAA; and
(2) A powered
parachute or weight-shift-control aircraft may be operated at less
than the minimums prescribed in paragraph (c) of this section.]

c) the aircraft may not be operated closer than 500 feet to any...structure. d1) except provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA. Are there examples of what "routes or altitudes" are prescribed by the FAA?

Yes there are heli charts for major metro areas, like NY and LA http://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/vfr/#helicopter

Also, according to this answer,  14 CFR 91.119 says about issuing a waiver: When issuing the waiver, the altitude requested should be an absolute minimum, but may not be less than 500 feet from persons or property, unless necessary to safeguard human life

i see they can be viewed in skyvector as well, just not easy to find https://skyvector.com/?ll=40.773711032655235,-73.99957660822189&chart=200&zoom=5

Interesting. I had no idea that weight-shift aircraft and powered-parachute aircraft were exempt from the basic 500' rule.  That inspired me to check FAR 103 re ultralight vehicles and I see that no 500' rule is listed there either, nor any 1000' rule, but flight over congested areas is prohibited at any altitude.  Interesting.

authors: אפיק בן ארצי


text:
Are there any air navigation uses for the HF frequencies?
I'm busy revising for a license conversion... Out of curiosity, does anyone know of a radio navigational aid that used the frequencies we call HF (2-25 MHz)?

What have you found so far? Gee and LORAN come to mind.

Hadn't found anything - "UnrecognisedFalling Object" has set me right

Current airborne radionavigation systems use MF (NDB), VHF (VOR/LOC, marker beacons), UHF (G/S), and L-band microwave  (GPS/GNSS, DME) frequencies.  (MLS operates in the C-band, and airborne radar in the X-band, but those are niche applications.)
Historically, the only system that used HF was the UK Gee system.  LORAN-A had options for the use of HF frequencies, but these were never used, as MF operation just below the 2 MHz mark gave much more consistent results in both reception range and performance, and LORAN-C migrated into the LF band at 180kHz, partially to avoid the difficulties of being co-located with the amateur radio 160m band.  Decca also used LF frequencies, and the Omega system operated at VLF to provide (imprecise) global coverage.
Furthermore, the FCC frequency table shows no allocations for the Radionavigation service anywhere from 2 to 30MHz, in any ITU region.

Comm related - HF frequencies are used for transatlantic/transpac flights out of range of VHF/UHF Comm.


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What is the probability that passengers can survive a ditching in a 747 at MTOW?
Let's suppose a catastrophic EFATO has occurred while climbing out of JFK from Runway 31L, the aircraft is at its MTOW and the best option is to set it down in Jamaica Bay. How long would the aeroplane float and what are the chances of surviving?

Well, I'm not sure if an "all engines flameout shortly after rotation" scenario would be the precipitating event I'd choose for this, but the overall concept is sound -- something roughly similar to the type of failure that caused N707AR to overrun the runway could do it...

See also http://www.airliners.net/aviation-forums/general_aviation/read.main/2426354/. Does not look like they reached any conclusion.

Aircraft are designed with ditching in mind, but the outcome depends on many circumstances, most importantly on how calm the water is and how well the aircraft can be controlled.
The Wikipedia article on Ditching lists emergency water landings and other accidents where aircraft ended in water.
There is no ditching by 747, so we don't have any specific reference. However:

B747 is not that different from other types of aircraft. Controlled ditchings on calm water were successful in different types; it is likely that one would be possible with B747 too.
How long the aircraft would remain afloat depends on the damage. But if it did not break up, it would likely manage to keep afloat for the 90 seconds needed for evacuation. And the slides can be used as life rafts.
There was one case where B747 ended up in water after overrunning runway (so at lower speed than landing), China Airlines flight 605, and the aircraft did remain afloat long enough to evacuate it.
Also any aircraft that will be used for flying across large bodies of water, which obviously includes all variants of B747, must be certified for ditching. This involves designing it so that, at least according to model, a controlled ditching will be smooth enough not to cause serious injuries to occupants and will leave it afloat long enough for successful evacuation.

In China Airliners flight 605, the plane did not sink because there is a shallow seabed. The runway is built on reclaimed land; and the airplane was close enough from the edge of the runway to be supported by the seabed.

@kevin: The wikipedia page says “The passenger cabin remained completely above water during the evacuation, although eventually sank tail-first.”, indicating that it floated for some time while it had where to sink. I didn't hunt any further. 1-2 minutes is all that is needed for evacuation.

@Jan: The official accident report confirms it floated (there's a PDF of a scanned copy). The report says It floated initially, winds blew it back towards shore, some time after evacuation it settled in a slightly tail-down position supported on main landing gear on sea floor, however no part of fuselage was  in contact with sea floor. Rear of lower passenger cabin was flooded/underwater.

The impression I got from reading the accident report for US Airways 1549 is that the "design" for ditching of large aircraft consists mostly of wishful thinking: there was one serious study of the ditching performance of a large airliner in (I think) the 1960s, and everyone since then simply assumes that their airliner will behave similarly upon contact with the water.

@Mark: To an extent, yes. The design is based on models and there is limited data to verify those models.

Regarding China Airlines 605: They blown up the vertical stabilizer with dynamite? lol

Concerning floating: planes tend to float for a long time, potentially indefinitely. The reason for this is that they are much lighter than water. For example, jet fuel is about 80% of the density of fresh water, so even if the wings were fully loaded with fuel, they would be lighter than water. Some of the space in the wing tanks is air, which of course is even lighter. There are vents in the tanks, but it would tank a long time like many hours or probably days before a wing could even partially fill up with water.
Concerning survivability: there is no way to predict the outcome. It would depend completely on the reactions and skill of the pilot and exactly when the engines lost power. In general, landing on water is not a problem, as was seen in the accident of US Air 1549. A big plane can (and has) actually run over a small house and stay relatively intact. So, even if it went up one of the beaches on the spit it might be OK. The biggest threats are running at high speed into an object or terrain feature, or stalling; both of those not happening are largely dependent on the skill and cool nerve of the pilot.
When aircraft take off, they usually have a steep rate of ascent which would be a stall configuration in unpowered flight, so if the engines fail the pilot has to immediately react to avoid a stall. When this happens, the pilot has to react within a fraction of a second or so. If the pilot panics and pulls back on the stick, it's game over.
If the pilot is good, they will have an "emergency plan" and will mentally know ahead of time exactly what they will do if they lose power at any given point on any given runway. That plan includes a landing spot. For example, when I fly to a new airport I look at Google Earth and decide where I will land in the case of an emergency during take off. The plan sometimes has to have height thresholds, too. For example, if I lose power at 100 feet I do this, but if lose power at 400 feet I do that. The plan always has a "minimum turnaround altitude". This is the height where you are safe to do a 180-degree turn and go back to the runway.  Note that the plan has to recalculated every time you fly because of wind. So, if there is a crosswind coming from one direction or another that could affect your choice of emergency landing location.


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Are all EGPWS-equipped aircraft using the same underlying system?
All modern transport-category aircraft and many other aircraft are equipped with enhanced ground proximity warning system (EGPWS). When this EGPWS acronym is used, is it always referring to the same system which has been integrated into the aircraft? Or is it a general term which refers to a system of various manufacturers?
To take it a step further, is there only a single system which allows aircraft to comply with the requirements to have this system integrated?

I'm pretty sure that EGPWS is a Honeywell trademark and TAWS is the generic acronym.

Yes and no.  When you say EGPWS that is a trademark of Honeywell, so all EGPWS systems are made by Honeywell.  The fine print is the the requirement TSO-C151c is for a Terrain Awareness and Warning System (TAWS).
The TSO functionally describes the Honeywell system. (Honeywell actually designed it before the FAA decided it was a necessary addition to the rules.)  Since implementing the standard would most likely require the use of Honeywell proprietary information (patents), Honeywell is legally required to provide the product it to all customers on a non-discriminatory basis.
As a result, even though Rockwell Collins provides the Integrated Surveillance System (ISS) for the B787, the TAWS component of the ISS is sourced by Collins from Honeywell.
That said, Thales was able to patent a similar system in France about the same time.  There were lawsuits, and in the end they agreed to let each other exist.  ACSS (a joint venture between L-3 Comm and Thales) sells TAWS units in the US that is based on the Thales system, so there is an alternative to Honeywell. Thales does sell their system outside the US.
Some of the primary Honeywell patents for the EGPWS have recently expired or are about to expire. So there may be some new alternatives soon.  Of course Honeywell has been making 'upgrades' beyond the regulatory requirements which they have been patenting.  These may help them hold their market share.
So no, there is not a single system to meet the requirement.  OTOH, there's not much competition either.


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Why do SIDs include an MEA?
Let's say we're flying the JORJO2 SID (1,2) out of KPWK (RBS transition). We are told "climb via the SID". Page 2 of the SID says "maintain 3000" but the MEA on page 1 is 4000.
Is there ever a time we are supposed to climb to the MEA?  Or are we to maintain the top altitude in the SID (3000) until specifically cleared higher?  I'm just not sure what the purpose of displaying the MEA is on this chart, lost comms maybe?


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List of airlines and aircraft types at Copenhagen Airport (CPH)
Is there a website with a list of all the airlines that fly to Copenhagen Airport (CPH), and with what aircraft types they fly to Copenhagen?
I have found a list of airlines at the website from Copenhagen Airport, but no cargo airlines are mentioned there, and also no aircraft types. I have started with that list and typing over the aircraft types from the complete fleet from Wikipedia or the website of the airline itself, and then looking at Flightradar 24 which aircraft of the fleet is actually used to fly to and from CPH, but it takes a really long time and due to summer/winter schedules I will probably miss out on some.
Or if anyone has tips for a faster (and more accurate) solution if such a list does not exist...

This can be obtained from the Eurocontrol Data, if you are a student they might allow you to acces the data:
https://ext.eurocontrol.int/
You have to request for an account there.


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Why so much ground time at Frankfurt Airport?
Disclaimer: Total layman here.

I frequently fly into and out of Frankfurt Airport (FRA) with regional jets (flight time advertised as 45 minutes) and one thing that everyone is always annoyed about is that you spend approximately the same amount of time in transit on the ground in Frankfurt as you spend in the air.
I understand FRA is a huge airport, but what does not make any sense to me is that you...

...have to walk 10 min to the gate
...must spend 10 min on a bus to get you to the aircraft
...then spend at least 10-15 min on the aircraft as it taxis to its start position on the runway

Given that these regional connection flights are always on a tight schedule, I do not quite get why they do it this way.
Specifically, can anyone explain why — in FRA — these small jets are parked so far away from their landing/starting positions when you have to get there by bus anyway?

Hm: https://aviation.stackexchange.com/questions/9500/why-does-schiphol-airport-have-such-a-long-taxi
... seems this is a near dupe wrt. to a different airport.

The "driving" is called taxying.

Taxiing, rather.

The regional connecting flights may be on a tight schedule, but remember, the airlines and especially the ticketing agencies are well aware of this. They won't sell you a ticket on any 2 given flights where the connection time between the two is less than the standard minimum connection time for a given airport. Yes, you may have to hustle to get from gate A to gate B to make your connection, but the airline doesn't want to have you miss your flight - rebooking you costs money and creates other issues for them, too.

@FreeMan I think the asker means that the plane must arrive and depart on time because they're used intensively on many flights per day, not that individual passengers might have tight connections.

@DavidRicherby - obviously not how I read it, but entirely plausible...

@TannerSwett Pretty sure taxying/taxiing are alternative forms, not wrong/right respectively..

Where are you flying to if I may ask? If you only fly 45 minutes and have to wait a long time before and after the flight, it might be faster to take the train.

@EricDuminil You may: FDH. Schwäb'sche Eisenbahn doesn't really cut it. :-(

If you want a long taxi, try charles de gaulle

I've recently experienced the same thing on Schiphol, but to me the things that you mention seem pretty obvious: it's all down to economics.
There is only so much parking space around the airport terminals. Aircraft for regional routes are smaller (50-120 passengers), so using a few buses to shuttle the passengers around is only a small undertaking. This allows the aircraft to be parked farther away from the terminals, and I can imagine that this decreases airport fees for the flight.
The regional flights mainly have competition from trains, buses and car travel. Using an airport takes up a lot of time (you didnt even mention luggage check-in and security), and you have to move around a lot. But all in all people still choose air travel because it is faster/cheaper/less of a hassle than other modes of transport for that specific route.

What you write with the sapce around the terminal make much sense, but not my final question above: I would find it interesting why the airplanes are parked so far away / require so much driving around from the runway, when the pasgrs are ferried over by bus anyway. Would seem more efficient to have the bus drive further so that the taxiing of the plane could be shorter.

Where should the aircraft parking be located according to you? Next to the runway? And which one? Airports like Frankfurt and Schiphol have many runways and which one are in use may change depending on wind direction and noise restrictions. And you don't want passenger aircraft to be parked at the other side of the airport, because you want as little traffic as possible crossing the runways and taxiways.

Schiphol has its own unique problem: the terminal was originally built near the old runways, but the new and preferred runway (Polderbaan, 18R, 3800 m) had to be built several kilometers from the terminal. If Schiphol would choose to uses buses for LCC's (like Frankfurt does), the problem would be that LCC's would have a major time advantage! Buses would be much faster than taxiing planes around a crowded airfield like Schiphol.


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Which radio altimeter is the Boeing 737NG call outs based on?
I'm wondering which of the two Radio Altimeters: Captain (left) or F/O (right), that the 737 does the RA callouts based on.
For instance, if the F/O's RA is slightly off, and passes 500ft before the captain's RA, does the aircraft call "500" when the first (in this case the F/O's RA) RA passes that altitude, or is it always based on the captain's RA?
Also, while on this topic, could someone confirm if the "Minimums" callout is always based on whatever is set on the Captain side, because to my knowledge, that is the case.

The FCOM (volume 2, 15.20.17) states:

The GPWS provides height callouts based on altitude set by the
  Captain's Minimums selector.

According to the investigation of Turkish Airlines 1951, the captain's and FO's radio altimeter are feed independently, and the autopilot is wired to react to whatever is happening on the captain's side. This includes the aural warnings.
Curiously, I cannot locate anywhere in the FCOM, FCTM nor QRH what happens when the RAs disagree. The only related information I find is, to use the autopilot for landing you'd need both RAs to be functional. If one of them fails while the autopilot is in landing mode (i.e. glideslope and/or localizer mode), the autopilot disconnects.

It provides height callouts based on Captain's minimums selector, but does it use left RA? I suspect it actually uses built-in GPS (because EGPWS is GPS-based)

What happens when the RAs disagree is embedded in your last statement. The safety assessment considers two failure types. The first is 'loss of function' which means you're getting no data from the RA or the RA data is set to Fail Warn by the RA's internal Continuous BIT. The other failure type is 'Hazardously Misleading Information' or HMI. In this case you receive what looks like good data, but it's wrong. The protection for this is the use of two RAs. The FCC will compare the data from the two and if they differ by some set amount, the FCC declares a failure.

@JanHudec It uses the RA.  EGPWS uses GPS for Forward Looking Terrain Avoidance. It still uses the RAs and Air Data for GPWS Modes 1 through 5 and altitude callouts.

the autopilot is wired to react to whatever is happening on the captain's side. This includes the aural warnings. This is not correct. There are two autopilots, each fed by the corresponding RA. There is a selector switch to use left, right or both autopilots. The autothrottle system is seperate and only uses the captain’s RA. If he captain’s RA reports unreliable data, it will automatically switch to the FO’s RA. Only the later systems on the 738 have a comparator that prevents retard mode if the RA’s disagree. Nothing about the callouts is indicated in the TK1951 report

to use the autopilot for landing you'd need both RAs to be functional This is also incorrect. In the TK1951 accident only the right AP was selected. It uses the FO’s RA, so operated correctly for the accident flight. Had it been the FO’s RA malfunctioning and they had selected the captain’s AP things would have operated normally. Or, if the captain’s malfunctioning RA reported its data to be unreliable the AT would have switched to the FO’s RA and operated normally. It only requires both RA’s to be working to select both AP’s for landing.


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What aviation-related TV shows are there?
This question aims to serve as a Community Wiki resource.
Feel free to edit the list of aviation-related TV shows below!
Related: What aviation-related movies are there?

Meta discussion simultaneously started: Are these questions on-topic? TV shows/movies

Disasters

Air Crash Investigation (a.k.a. Mayday): 2002 - now -
Aircrash Confidential: 2011 - now -
Seconds From Disaster: 2004 - now - not all aircraft related
Terror In The Skies: 2013 - now -

Documentaries

Airplane Repo: 2010 - now -
American Fighter Pilot: 2002 -
Dangerous Flights: 2012 - now - follows a team of ferry pilots
Flying Wild Alaska: 2011 - now -
Jet Stream: 2008 -  follows 8 pilots training with the Royal Canadian Air Force to fly one of the most advanced supersonic tactical fighter jets in the world — the CF-18 Hornet
Ice Pilots:  2009 - now -
Mighty Planes: 2012 - now - series about unique large aircraft
The Aviators: 2010 - now -

Airports

Airport 24/7: Miami: 2012 - now - follows the daily life at Miami International Airport
Animal Airport: 2000 - now -
Ultimate Airport: Dubai: 2013 - now - follows the daily life and construction at the world's 2nd busiest airport1

Sitcoms

Air America
Come Fly With Me
Wings: 1990 - 1997 - drama/comedy

Dramas

AirWolf
Black Sheep Squadron
Blue Thunder
El Señor de los Cielos: 2013 - now - Spanish language, English subtitles
LAX
Pan Am
Pensacola: Wings of Gold
SuperCarrier